One of the issues facing the aging society that is gaining increasing attention involves problems concerning the bones, such as the changes in bone density or morphology accompanying the aging of body function, and damages to the bone due to falling accidents or the like. It is particularly important to overcome such problems to achieve “safe, secure, and high-quality life in an aging society.”
For example, when an orthopedic implant such as an artificial joint is attached in the event of damage to a bone, the implant attached may cause a dynamic environmental change. This could, under certain conditions, induce a change in the density or morphology of the bone, resulting in the loosening of the implant. Therefore, it is extremely important to monitor the stress distribution in the bone to which the implant is attached as accurately as possible if the attached implant is to maintain appropriate biocompatibility for a desired period of, for example, 20 years or longer.
For the analysis of bone stress distribution, conventionally the strain gauge method has been used whereby, particularly for the measurement of surface stress distribution, a number of strain gauges are affixed to the surface of a bone so as to analyze stress distribution based on the stress applied to the bone and the output signals from the individual strain gauges. However, the number of such affixed strain gauges is limited and it has been impossible to conduct measurement without omitted points.
As a countermeasure, the thermoelastic stress measurement method has recently been developed by which surface stress is visualized on the basis of the surface temperature of a sample, which varies depending on stress. This technique, based as it is on infrared thermography, makes it possible to visualize the surface stress distribution in a femur, for example, to which an artificial joint is attached, without omitted points.
Meanwhile, the applicants have conducted studies on inorganic materials that emit light in response to application of mechanical energy, and have successfully manufactured a material from inorganic substances consisting of a base material that is a piezoelectric body with wurtzite type structure, in particular, and a luminescence center, as disclosed in Patent Document 1. The applicants have found that when a luminescence center is added to the base material, the light-emission intensity of an obtained thin film can be significantly increased. The applicants have filed a patent application for this discovery. Subsequent studies have led to the discovery of a variety of inorganic materials that emit light based on the force, and studies are also underway to utilize such materials in various fields. This is disclosed, for example, in Patent Document 2, in which detection of abnormal stress prior to the destruction of concrete by mixing a mechanoluminescence material in the concrete is proposed.    Patent Document 1: JP Patent Publication (Kokai) No. 11-120801 A (1999) (U.S. Pat. No. 3,265,356)    Patent Document 2: JP Patent Publication (Kokai) No. 2003-137622 A